Etching liquid (4)
I believe the answer is:
acid
'etching liquid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acid that I've seen before include "something sharp" , "Bitter or sour" , "Opposite of alkali; sour" , "Etcher's fluid" , "Sour, sharp-tasting" .)